Coating of molybdenum with binary coatings containing aluminum



Dec. 4, 1956 E. WAINE COATING OF MOLYBDENUM WITH BINARY COATINGS CONTAINING ALUMINUM Filed Aug. 8, 1951 PURGING GAS PURIFICATION flYDROGEN PURIFICATION HYDROGEN PURIFICATION ALUMINIUM COMPOUND A SECONDARY COMPOUND Waz'lzer 7: 5 wQ W4H 7 5 United States Patent COATING OF MOLYBDENUM WITH BINARY COATINGS CONTAINING ALUMINUM Eugene Wainer, Cleveland, Ohio, assignor to Thompson Products, Inc., Cleveland, Ohio, a corporation of Ohio Application August 8, 1951, Serial No. 240,862

' 3 Claims. (Cl. 117-71 The present invention relates to a method of coating refractory metal articles to increase their resistance to high temperatures and corrosive atmospheres. The process of the present invention is particularly applicable to the manufacture of coated molybdenum turbine buckets for use in jet turbines and the like.

Turbo-jet engines and the like are usually provided with an axial flow turbine operated by exhaust gases which drive a blower furnishing air to the burners. Such tur bines operate at extremely high temperatures, and one of the major difiiculties encountered in the manufacture of jet turbines has been the provision of suitable material for turbine buckets which can withstand the effect of such high temperatures. Turbine buckets are frequently exposed to temperatures in the range from 1600 to 2000 F. and above, and the bucket must have sufficient strength, toughness, creep resistance, and resistance to the corrosive atmosphere present to enable the bucket to operate efiiciently without deformation or corrosion.

In addition to turbine buckets, articles produced by the present invention may be employed under conditions of higher temperatures and lower stress than exists in a gas turbine bucket. One such application occurs in nozzle diaphragm vanes in gas turbines which must withstand very severe conditions of temperature and thermal shock but ata relatively lower stress.

One metal which exhibits excellent properties of strength, toughness and creep resistance at elevated temperatures is molybdenum. However, metallic molybdenum cannot be used in high temperature environments which contain oxidizing gases because molybdenum readily oxidizes to form a sublim able oxide which causes the complete disintegration of molybdenum bodies in oxidizing atmospheres at temperatures even below those which occur in the operation of a gas turbine engine.

To make molybdenum articles available foruse in such high temperature environments under oxidizing conditions, the present invention provides a process for coating molybdenum bodies to provide a tough, corrosion-resistant coating integrally bonded to the molybdenum body and impervious to the effects of oxygen and other gases.

In the present invention, molybdenum articles are provided with a binary coating of aluminum and another element selected from the group consisting of boron, silicon, titanium and zirconium.

An object of the present invention is to provide a method forwcoating' molybdenum bodies to produce an extremely corrosion-resistant surface thereon.

Another object of the present invention is to provide a method for coating molybdenum with a firmly bonded oxygenrimperviou's barrier. I

In the process of the present invention, the molybdenum article istreated at its surface with aluminum, and the aluminum is reacted with another element such as boron, silicon, titanium, or zirconium to produce a binary coating. The process of the present invention may be carried out in several distinct manners. The preferred ice process involves first depositing aluminum on the molybdenum body by means of a vapor phase deposition process, and subsequently reacting the coating thus applied with one of the metals or metalloids previously mentioned to produce the binary coating.

The elements such as boron, silicon, titanium and zirconium, have ionic sizes which are close enough to the ionic size of aluminum to allow the two elements to be mutually soluble with each other in the solid state, and thus increase the rate ofwintermetallic compound formation. Such intermetallic compounds per se, or the compounds formed with the base metal, molybdenum, then have a size which approximates the atomic spacing in lattice of the molybdenum. The primary coating of aluminum inherently leaves microscopic voids, tunnels, or weak spots in the surface of I the article, thus decreasing its ability to withstand corrosion. By reacting the aluminum coating with a second element of the type mentioned, it is believed that the voids in the atomic spacing of the aluminum coating are filled by the reaction with the second element, thus making the aluminum layer much less permeable.

In carrying out the coating process of the present invention, a decomposable compound, preferably a halide of the coating metal is carried into a reaction zone in a stream of hydrogen or other reducing gas and therein dezone is the metathetical reaction between the coating aluminum coating.

metal compound and the molybdenum wherein the coating metal is deposited on the molybdenum with the formation of a volatile molybdenum compound in the exchange reaction.

As a source of aluminum in the vapor phase deposition process, I prefer to use an aluminum halide such as aluminum chloride, aluminum bromide, or aluminum iodide. Since these compounds are normally solid, they maybe introduced into the coating zone by passing hydrogen gas over "a heated supply of the compound in powder form to carry the compound into the coating zone. v I

In reacting the aluminum coating with the second element, a decomposable compound capable of yielding the second' element is introduced into the reaction zone either prior to, during, or after the deposition of the primary Typical among the decomposable compounds which maybe used for this process are boron t'ricliloride, boron tribromide, boron triiodide, titanium tetrachloride, titanium tetrabromide, silicon tetrachloride, trichlor'o silane, silicon tetra'bromide, tribrom'o silane, silicon tetraiodide; zirconium chloride, zirconium bromide, and zirconium iodide.

The reaction conditions in the coating zone include temperatures from 1600 to 2200 F., and preferably on the order of 2000 F., for periods of time ranging from one to eight hours, depending upon the thicknes of the coating desired, and the penetration required. At a treating time of four hours, for example, coatingsranging from 0.0002 inch to 0.0032 inchthick, depending upon the temperatures employed, have been produced. The rate of increase in deposition thickness increases with the temperature.

Aluminum reacts with a metalloid such as boron, silicon, titanium and zirconium to produce a series of in termetallic compounds with the molybdenum metal. Such compounds range from materials having a high proportion of molybdenum, such as MosAl, to materials having substantial amount of aluminum, such as MoAli. The presence of these interrnetallic compounds has been found instrumental in achieving the desired corrosion resistant properties.

The intermetallic compounds of aluminum and silicon, as well as the solid solutions of these two metals are stable at high temperatures, and provide an excellent integral bond with the molybdenum base. The reaction of the aluminum coated molybdenum body with an element of the type described enhances the properties of the aluminum coating by filling up the microscopic voids and Weak spots, resulting in a substantial increase in stability and resistance of the coated article to high temperature oxidation.

A further description of the present invention will be made in connection with the attached sheet of drawings in which:

Figure 1 is a flow sheet showing in a diagrammatic manner the various stages of the coating process; and

Figure 2 is a drawing of a photomicrograph taken at a magnification of about 500 X showing the crystal structure of a molybdenum article coated in accordance with the present invention.

As shown on the drawings:

In Figure 1, reference numeral denotes a supply of purging gas which is preferably an inert gas such as nitrogen, argon, neon, helium, or the like which is passed into a purification zone 11 Where moisture or other contaminants are removed. The purification zone 11 may consist of a supply of liquid sulphuric acid through which the purging gas is bubbled. The purified purging gas is next introduced into a heated furnace 12 which surrounds a furnace tube 13, control of the gas flowing into the furnace tube 13 being controlled by means of a valve 14.

Disposed within the furnace tube 13 are a plurality of boats 15 which carry a number of turbine buckets 16 or similar articles composed of molybdenum. These articles are normally pre-shaped into their desired form, and worked at temperatures below the recrystallization temperature of molybdenum, to enhance their physical properties. The temperature of the furnace 12 is regulated between 1600 and 2200 F., with 2000 F. being the preferred value.

A supply of hydrogen gas 17 is provided for introduction into the tube 13. Prior to its introduction into the furnacing system, the hydrogen gas'is dehydrated and purified by means of various desiccants such as, for example, packed columns of silica gel, calcium chloride, or liquid sulphuric acid in a purification stage 18. The flow of the purified hydrogen gas into the furnace 13 is controlled by means of a valve 19.

A second source 20 of hydrogen gas and another purification stage 21 is also provided to furnish a carrying medium for the coating compounds. A source of aluminum, such as powdered aluminum chloride, is maintained in a zone 22. A source of the secondary metal of metalloid, preferably a halide of the type mentioned previously, is contained in zone 23. The flow of hydrogen into the zones 22 and 23 is regulated by means of valves 24 and 25 while fiow of hydrogen gas containing the decomposable coating compounds is regulated by means of valves 26 and 27. In this arrangement, either of the coating compounds may be introduced separately into the reaction zone, or the introduction of both compounds may be made simultaneously.

Initially, the furnace tube 13 is purged by means of the purging gas to eliminate from the furnace chamber moisture, oxygen, and other undesirable contaminants. After the purging, the hydrogen and the coating metal compounds are introduced into the furnace tube 13, where they contact the molybdenum articles 16 for periods of time ranging from one to eight hours. Excess hydrogen is vented from the furnace tube by means of a tube 28.

The molybdenum articles 16 are maintained in the furnace tube 13 until a coating having a thickness of approximately 0.0002 to 0.004 inch is produced on the molybdenum article.

Figure 2 illustrates a photomicrograph of a molybdenum article treated by simultaneous deposition of aluminum and titanium for a period of seven to eight hours at a temperature of 2000 F. As illustrated in Figure 2, the body of the article consists of rather large crystals of molybdenum 29 having an overlying layer of aluminum 30 integrally bonded thereto. Immediately above the aluminum layer 30 is a layer 31 of indeterminate composition, which is probably a complex mixture of molybdenum, aluminum, and titanium in the form of various intermetallic compounds. The outer uppermost layer 32 is a mixture of aluminum and titanium compounds, and solid solutions of these two elements. This structure has been found to be capable of withstanding operation of the gas turbine operating at temperature estimated at 1600 to 1800 F. for periods in excess of hours without apparent deterioration.

From the'foregoing, it will be appreciated that I have herein provided a process for coating molybdenum bodies which are in themselves incapable of withstanding the corrosive effects encountered in the operation of the gas turbine.

The coatings produced in accordance with this invention are integrally bonded to the base metal and cannot be stripped mechanically from the molybdenum, as is the case of coatings applied by electroplating or dipping. Further the coatings are inert with respect to the molybdenum base metal and show no evidence of reaction with the base metal after the original deposition.

The vapor phase deposition processes described herein have complete throwing power i. e., a uniform coating can be deposited over the entire surface of the article regardless of corners, grooves, or other surface irregularities. This is not true when using other types of coating procedures.

The present application is a continuation-in-part of my copending application Serial No. 103,632, filed July 8, 1949, entitled Binary Coating of Refractory Metals, issued on Sept. 28, 1954, U. S. Patent No. 2,690,409.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. The method of providing a corrosion-resistant surface on a molybdenum article which comprises depositing a layer of aluminum on said article, and thereafter reacting the resulting coated article with an element selected from the group consisting of boron, silicon, titanium, and zirconium.

2. The method of providing a corrosion-resistant surface on a molybdenum article which comprises decomposing an aluminum compound at a temperature of 1600" to 2200 F. in an atmosphere of a reducing gas to form aluminum, depositing said aluminum on said article, and thereafter reacting the resulting coated article with an element selected from the group consisting of boron, silicon, titanium, and zirconium.

3. The method of providing a corrosion-resistant surface on a molybdenum article which comprises decomposin an aluminum compound at a temperature of from about 1600 F. to 2200 F. in an atmosphere of a reducing gas to form a primary coating of aluminum on said article, decomposing a vaporized decomposable halide compound of an element selected from the group consisting of boron, silicon, titanium and zirconium in an atmosphere of a reducing gas to yield said element, and reacting said element with said aluminum coating to form a corrosion resistant secondary coating on said molybdenum article.

References Citedihihfi file of this patent UNITED STAjES PATENTS Martin July 8, 1930 Austin Jan. 16, 1934 5 Sayles 4--. Ian. 17, 1935 Scheller Mar. 21, 1939 Morgan Oct. 16, 1945 Whitfield Dec. 7, 1948 Strong w.-. Dec. 21, 1948 10 Sears Dec. 13, 1949 Widell Jan. 2, 1951 

1. THE METHOD OF PROVIDING A CORROSION-RESISTANT SURFACE ON A MOLYBDENUM ARTICLE WHICH COMPRISES DEPOSITING A LAYER OF ALUMINUM ON SAID ARTICLE, AND THEREAFTER REACTING THE RESULTING COATED ARTICLE WITH AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF BORON, SILICON, TITANIUM, AND ZIRCONIUM. 